The basic principle of mass spectrometry is to generate ions, to separate these ions by their mass-to-charge ratios (m/z) and to detect them quantitatively according to their respective m/z ratios. A scanning mass spectrometer is arranged to allow only those ions of a (nominal) single m/z to be detected at any instant in time (as resolution allows). Over time, however, the control parameters of the separation can be varied so as to scan a sequential range of m/z ions. Scanning starts at a predetermined m/z and increments (or decrements) at a predetermined rate (scan rate) until a predetermined final m/z is achieved. The resulting detector output signal is monitored during the scan dwell time after each m/z increment (or decrement) and is subsequently recorded. This generates a record of ion intensity versus m/z (and time). The scan is then typically repeated for a specified and predesignated period of time. The initial m/z, final m/z, scan dwell and resulting scan cycle time are preset and fixed prior to scanning and acquiring data.
Alternatively, in another mode of operation of a scanning instrument, the control parameters can be fixed at a particular m/z for a set period of time. This analysis of a single m/z is called selected ion monitoring or SIM for short. Typically a specified group of m/z's is SIMed together in a sequence, which is called a SIM group. The SIM group measurement cycle begins by selecting the first m/z in the group. The operating control parameters are allowed to stabilize and the selected m/z ions are allowed time to traverse into the ion detector. The resulting detector output signal is then monitored for a specified period of time (referred to as the SIM dwell) and recorded. Next the control parameters are switched to values necessary to select the next m/z in the measurement cycle. This process repeats until all m/z's in the group have been monitored and recorded. The SIM cycle is then repeated continuously for a specified period of time. The SIM dwell for each m/z and the resulting SIM cycle time are also preset and fixed prior to SIMing and acquiring data.
For historical and technical reasons, the SIM dwell times and scan dwell time as outlined above are fixed prior to and during operation. Consequently, the scan cycle time and SIM cycle time are also predetermined and invariant. This deterministic approach makes the sequencing of events a straightforward proposition and relatively easy to implement. Data analysis is also simplified.